1. Field
The present invention relates to a semiconductor device, and a method for producing the semiconductor device.
2. Description of the Related Art
The wiring substrates on which one or more semiconductor elements are to be mounted are produced by arranging wiring of metallic material on one or more surfaces of an insulating substrate of, for example, a ceramic material. Generally, such wiring substrates can be produced by either a co-firing process or a post-firing process. According to the co-firing process, materials for wires are arranged before firing for producing the ceramic material. According to the post-firing process, wires are arranged after the firing for producing the ceramic material.
In many cases where a wiring substrate is produced by use of a ceramic substrate, a large collective substrate on which a number of semiconductor devices can be formed is produced in consideration of assemblability during a process of manufacturing the semiconductor devices. The post-firing process is used in many cases where wiring substrates are produced from a large collective substrate because the post-firing process is superior in precision in the patterning for wiring.
Electrodes can be formed by post-firing processes in two ways. In the first manner of electrode formation using a post-firing process, all the electrodes are formed by using a physical vapor deposition technique such as sputtering. For example, the ceramic wiring substrate disclosed in Japanese Patent Laid-Open No. H01-11394 has a wiring layer on a ceramic substrate, where the wiring layer is arranged by forming, on the ceramic substrate, a contact layer of titanium (Ti), a barrier layer of an alloy of Ti and tungsten (W), and a main conductive layer mainly constituted by copper (Cu), in this order by physical vapor deposition. The formation of the ceramic wiring substrate is completed by further arranging a noble metal layer of gold (Au), platinum (Pt), palladium (Pd), or the like for the purpose of antioxidation and improvement in solder wettability and wire bondability.
On the other hand, in the second manner of electrode formation using a post-firing process, an underlying layer for each electrode is formed by physical vapor deposition, and thereafter each electrode is formed by plating. In many cases, titanium (Ti) is used for forming an adhesion layer which constitutes the underlying layer and maintains adhesion to the ceramic substrate. In addition, in order to efficiently dissipate heat generated by the semiconductor elements, copper (Cu) is used as the plating metal in many cases. Further, formation of the wiring substrate of the above type is completed by growing a copper plating in positions in which electrodes are to be formed, removing portions of the underlying layer on which no electrode is to be formed and exposing the corresponding portions of the surface of the ceramic substrate and side surfaces of the underlying layer, and growing a plating of nickel (Ni), silver (Ag), gold (Au), or the like.
In the second manner of electrode formation using a post-firing process, the underlying layer the side faces of which are exposed after the growth of the Cu plating includes the adhesion layer for adhesion to the ceramic is formed of Ti. However, the plating of Ni, Ag, Au, or the like does not adhere to Ti. Even if the plating of Ni, Ag, Au, or the like adheres to the side surface of the Ti adhesion layer, part of the side surface of the Ti adhesion layer are still exposed or the plating of Ni, Ag, Au, or the like easily comes off. In other words, it is impossible to stably adhere the plating of Ni, Ag, Au, or the like to the side surfaces of the Ti adhesion layer. Therefore, gaps are produced between the plating of Ni, Ag, Au, or the like and the ceramic substrate. In addition, Ti is prone to be oxidized.
Further, generally, the semiconductor devices in which a semiconductor element is mounted on a wiring substrate are used in various environmental conditions. In particular, when the semiconductor devices in which a semiconductor element is mounted on a wiring substrate and a gap exists between the ceramic substrate and the plating are used in a high-temperature and high-humidity environment for a long time, the adhesion reliability is lowered. Specifically, in the high-temperature and high-humidity environment, water filtration into the gap between the ceramic substrate and the plating occurs, so that the film of titanium oxide becomes prone to corrosion. When the corrosion advances to a final stage, exfoliation occurs in the underlying layer under each electrode, so that the driving voltages of the above semiconductor devices become abnormal.